Examples of generally known infrared sensors for detecting a long wavelength infrared ray around 10 μm having very small energy, such as radiation from human body, include a pyroelectric sensor, a thermoelectric sensor, and a bolometer sensor that are thermopile sensors (for example, refer to JP06-201477A (hereinafter referred to as Patent Document 1)). Although the pyroelectric sensor is configured to generate an output voltage signal based on the temperature change of a pyroelectric material arranged on an absorber that absorbs infrared radiation, an output signal can be obtained only from a moving object or when the incidence of light is changed. On the other hand, the thermopile and the bolometer can output electrical signals proportional to a certain amount of infrared radiation, being able to be used in radiation thermometer or the like, and also for the human body detection.
One example of the radiation thermometer includes an in-ear thermometer. In the in-ear thermometer, a thermopile for detecting the infrared ray emitted from near the eardrum and a waveguide for guiding the infrared ray to the thermopile are arranged in the probe that is inserted into external auditory canal upon measurement. The output from the thermopile is converted to body temperature by computing means for display (for example, refer to JP3690387B).
Examples of inventions known to the public related to the infrared sensor other than the ones described above include inventions in JP05-191161A, JP2000-341055A, JP05-234120A and WO2005/027228A1 (hereinafter referred to as Patent Documents 2, 3, 4 and 5) described below. A configuration of one example of a method of extracting the electromotive voltage of photodiodes is described in Patent Document 2 where plural photodiodes are connected in series in multistage. According to such a configuration, an output that is a multiple of the number of photodiodes greater than the output of the electromotive voltage of each photodiode can be obtained, and thus, a large voltage can be obtained as a whole even if the electromotive voltage per photodiode is small. Patent Document 3 describes, as shown in its FIG. 5, an infrared sensor comprising a photodiode, an operational amplifier circuit, and a resistive element. The infrared sensor amplifies the signal of the current (electromotive current) generated in the photodiode and extracts the resultant as an output.
Furthermore, Patent Document 4 describes a light receiving element circuit comprising a photodiode applied with a reverse bias voltage and a differential amplifier that amplifies a photocurrent of the photodiode, wherein a photodiode biased with a reverse voltage is connected to one end of the photodiode. According to such a configuration, the output fluctuation of the photodiode can be reduced. Patent Document 5 describes an infrared sensor having a quantum infrared detection element in a light receiving unit, and InAsxSb1-x (0≦x≦1) is used on the light receiving surface of the infrared detection element.